1. Field of the Invention
The invention relates generally to electrostatic chucks for holding a workpiece and, more specifically, to a method for releasing a workpiece (such as a semiconductor wafer) from an electrostatic chuck.
2. Description of the Background Art
Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer process chamber. Although electrostatic chucks vary in design, they all are based on the principal of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece.
In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a support pedestal during processing. The support pedestal may form both an electrode and a heat sink as used in etching or chemical vapor deposition (CVD) applications. More specifically, the electrostatic chuck has a layer of dielectric material covering a conductive pedestal base. In a "unipolar" electrostatic chuck, voltage is applied to the conductive pedestal base relative to some internal chamber ground reference. Electrostatic force is established between the wafer being clamped and the electrostatic chuck. When the voltage is applied, the wafer is referred back to the same ground reference as the voltage source. Alternatively, the plasma can reference the wafer back to ground, although some voltage drop occurs across plasma sheaths that form at both the wafer being clamped and the reference electrode.
To cool the wafer during processing, an inert gas such as helium is pumped between the wafer and chuck surface so as to act as a thermal transfer medium to the water-cooled chuck. With helium pressure applied at the wafer-to-chuck interface, a relatively high electric field magnitude is required to generate the necessary electrostatic force required to clamp the wafer. For example, an electrostatic chuck using an alumina dielectric (a dielectric constant of approximately 10) that is operated at a heat transfer gas pressure in the range of 15 to 30 torr requires an electric field magnitude in the range of 0.5.times.10.sup.5 to 1.times.10.sup.5 V/cm. The electric field in a vacuum gap between the wafer and the chuck is higher by the ratio of the dielectric constants of the dielectric material and vacuum. In this example, the ratio is approximately 10 to 1; thus, the electric field in a vacuum gap can be ten times greater than the electric field at the surface of the wafer. Specifically, the resultant electric field can reach 1.times.10.sup.6 to 5.times.10.sup.6 V/cm in the vacuum gap.
Such high electric field magnitudes can cause charging of the dielectric material that may inhibit dechucking of the wafer after processing. In an ideal dielectric, i.e., bound charges only, the high electric field can cause charging of the surface between the wafer and the chuck. The mechanism causing this charging is field emission charging that is made possible by the high electric field reducing the potential barrier height, i.e., overcoming the work functions of the wafer and/or the chuck surface materials, allowing charges to tunnel from the wafer to the chuck surface or vice versa. The amount of charging caused by field emissions depends on the work functions of the surface materials concerned and the effective electric fields at the interface between the surfaces. The induced electric field resulting from field emission charging always opposes the applied electric field, reducing the net electric field and the resultant electrostatic force between the chuck and wafer. However, when the applied electric field is removed, i.e., the chuck voltage is set to 0 volts, a residual electrostatic force remains due to the presence of charge on the surface of the chuck and on the wafer backside surface as a result of the field emission charging. Consequently, dechucking of the wafer may be difficult because the residual electrostatic force must be overcome by a mechanical wafer lift mechanism to remove the wafer from the chuck.
The high electric field magnitude can cause other charging behavior that leads to additional dechucking difficulties. For example, if the chuck dielectric material is not an "ideal" dielectric, i.e., some unbound charge or "free charge" is present, then the free charge can drift through the dielectric material under the influence of the applied electric field. The free charges migrate toward the interface between the chuck and the wafer, producing an increase in free charge density near the surface of the chuck. Depending on the contact resistance between the wafer backside and the chuck surface, the charge may not be able to cross the interface. The result is an accumulation of charge within the dielectric material close to the surface of the chuck dielectric. This charge migration phenomenon depends upon material, time, temperature and electric field magnitude. The result is an effective increase in the electrostatic force at the wafer-to-chuck interface over that of the ideal dielectric. Since both polarization charges and free charges contribute to the electrostatic force, the force increase occurs due to the effective reduction in the separation distance between charges on the wafer backside and charges in the dielectric. However, when the applied electric field is removed, i.e., the chuck voltage is set to 0 volts, a residual electrostatic force remains due to the presence of migrated charges within the chuck dielectric and on the wafer backside surface. As such, the residual electrostatic force increases the physical force necessary to dislodge the wafer from the chuck.
The above effects are two examples of charging phenomenon that occur during electrostatic chuck operation that can affect the ability to dechuck a wafer. In fact, both charging phenomenon can occur on a particular electrostatic chuck depending on the conditions under which the chuck is operated. However, field emission charging and charge migration charging produce competing effects, i.e., field emission charging produces an electric field opposing the direction of the applied field and charge migration charging produces an electric field directed in the same direction as the applied field. For example, a unipolar chuck comprised of a plasma sprayed ceramic such as Al.sub.2 O.sub.3 /TiO.sub.2 dielectric operated in a high density plasma etch process at low temperatures (e.g., -10 degrees Celsius) has a dominant field emission charging phenomenon. Field emission charging reduces the electrostatic force during processing and leads to a residual electrostatic force after removing the applied electrostatic chuck voltage. If the wafer is somehow removed from the electrostatic chuck, and if another wafer is processed using the same chuck, an additional charging of the dielectric occurs. This cumulative charging effect continues until charge saturation occurs at some charge level that depends on the environmental conditions used during processing of the wafers.
Alternatively, if the same chuck is operated at a higher temperature (e.g., +30 degrees Celsius), under the same environmental conditions, then charge migration through the dielectric dominates the residual charging process. The high charge density at the wafer-to-chuck interface that results due to charge migration serves both to increase the applied electric field and to neutralize, to some degree, field emission charging at the chuck surface. The effective electric field and electrostatic force between the electrostatic chuck and the wafer is increased by the charge migration charging effect. However, once the applied chucking voltage is removed, a residual electrostatic force results which inhibits the dechucking of the wafer from the chuck. Because this charging depends on time, electric field strength and temperature, the residual effects are unpredictable. Additionally, the residual charge and force increases from wafer to wafer until some saturation condition is reached.
Consequently, there is a need for a dechucking method that is capable of dechucking a wafer from an electrostatic chuck under any environmental conditions and without damage to the hardware or the wafer being processed. Prior art techniques for dechucking the wafer include (1) setting the applied electrostatic chuck voltage to 0 and waiting some period of time for the wafer and electrostatic chuck to discharge; (2) setting the applied electrostatic chuck voltage to some other fixed voltage, waiting some period of time for the wafer chuck to discharge; (3) using either (1) or (2) followed by forcing the wafer off the electrostatic chuck by a mechanical means that overcomes the residual electrostatic forces; or (4) conductively connecting the wafer to ground by making ohmic contact between the wafer and ground. Methods (1), (2) and (3) suffer from unknown, and possibly very long, waiting periods. On the other hand, method (4) may not work on wafers with insulating backsides and may cause device damage to the wafers.
A significant shortcoming of the conventional approaches to removing the electric charge is that they fail to completely remove the charge such that some residual electrostatic force remains between the wafer and the chuck. This residual electrostatic force necessitates the use of a relatively large mechanical force to separate the wafer from the chuck. The force required for removal sometimes cracks or otherwise damages the wafer. Even when the wafer is not damaged, the difficulty of mechanically overcoming the residual electrostatic force sometimes causes the wafer to dislodge from the chuck unpredictably into a position from which it is difficult to retrieve using a conventional wafer transport robot.
Therefore, there is a need in the art for a method of dechucking a semiconductor wafer from an electrostatic chuck that minimizes the residual electrostatic force between the wafer and the chuck surface.